In software engineering, Unified Modeling Language (UML) is a non-proprietary, third generation modeling and specification language. However, the use of UML is not restricted to software modeling. It can be used for modeling hardware (engineering systems) and is commonly used for business process modeling and organizational structure modeling. The UML is an open method used to specify, visualize, construct and document the artifacts of an object-oriented software-intensive system under development. The UML represents a compilation of best engineering practices which have proven successful in modeling large, complex systems, especially at the architectural level.

UML integrates the concepts of Booch, OMT, OOSE and Class-Relation by fusing them into a single, common and widely usable modeling language. UML aims to be a standard modeling language which can model concurrent and distributed systems.

UML is an industry standard, created under the auspices of the Object Management Group (OMG). OMG had called for information on object-oriented methodologies, that might create a rigorous software modeling language. Many industry leaders had responded in earnest to help create the standard.

There are three prominent models of the UML system development:

Functional Model

Showcases the functionality of the system from the user's Point of View. Includes Use Cases Diagrams.

Object Model

Showcases the structure and substructure of the system using objects, attributes, operations, and associations. Includes Class Diagrams.

Dynamic Model

Showcases the internal behaviour of the system. Includes Sequence Diagrams, Activity Diagrams, Statechart Diagrams.

It is important to distinguish between a UML model, and a UML diagram, or set of diagrams, including Use Case Diagram, Collaboration Diagram, Activity Diagram, Sequence Diagram, Deployment Diagram, Component Diagram, Class Diagram, StateChart Diagram—a UML diagram is a graphical representation of the information in the model, but the model exists independently. XMI in its current version provides interchange for the model, but not for the diagrams.

UML uses a graphical notation which has text equivalents in Java and other object-oriented languages, and also ontological equivalents which are high-level enough to merit articles in Wikipedia. To show the degree of development of this language, it is possible to state concepts such as political processes in UML notation. Thus, it is possible to translate these schemas into executable programming languages.

UML is not a methodology and does not force one, but it is a requisite for the Rational Unified Process.

UML concepts

UML uses the following concepts:

• Actor
• Activity
• Interface
• Package
• Class
• Statechart
• Sequence
• Event

• IS-A
• HAS
• USES
• DEPENDS-ON

• There is a cardinality notation which corresponds to Database modeling cardinality, eg: 1, 0..1, 1..*

• There are specialized classes for common uses, such as Role, Use Case
• The concept of a stereotype is built-in to the language. It qualifies the symbol it is attached to.

Specification versions

There are several versions of UML specification. The current official version is UML 1.5. OMG is currently upgrading UML to Version 2.0 which differs from version 1.5 significantly. UML 2.0 is in finalization stage and is expected to be released soon. Many modeling tools vendors already support, up to some extent, UML 2.0

UML Diagram types

Use Case Diagram

This diagram describes the functionality of the (simple) Restaurant System. The Food Critic actor can Eat Food, Pay for Food, or Drink Wine. Only the Chef Actor can Cook Food. Use Cases are represented by an oval and Actors are stick figures. The box defines the boundaries of the Restaurant System, i.e. the use cases shown are part of the system being modelled, the actors are not.

The OMG defines a graphical notation for use cases; but it refrains from defining any written format for describing use cases in detail. Many people thus suffer under the misapprehension that a use case is its graphical notation; when in fact it is the written description of scenarios regarding a business task which provides the true value of a use case.

Class Diagram

This diagram describes the structure of a simple Restaurant System. UML shows is_a relationships with a triangle; and containers with diamond shape. Additionally, the role of the relationship may be specified as well as the cardinality (in the diagram above, the roles are accidentally on the wrong end of the lines—for example, seats should be at the association end that terminates on Patrons—see Fowler and Scott, UML Distilled, 2nd Edition, page 56 or Mellor and Balcer, Executable UML, page 214). The Restaurant System has any number of Food dishes, and one Kitchen, Dining Area, and any number of staff. All of these objects are associated to one Restaurant.

Sequence Diagram

This diagram describes the sequences of messages of the (simple) Restaurant System. This diagram represents a Patron ordering food, eating the food, drinking wine, then paying for the food. The dotted lines extending downwards indicate the timeline. The arrows represent messages (stimuli) from an actor or object to other objects. For example, the Patron sends message 'pay' to the Cashier. Half arrows indicate asynchronous method calls.

Sequence diagrams are functionally equivalent to collaboration diagrams.

Collaboration Diagram

A Collaboration diagram models the interactions between objects in terms of sequenced messages. Collaboration diagrams represent a combination of information taken from Class, Sequence, and Use Cases Diagrams describing both the static structure and dynamic behavior of a system. Collaboration and sequence diagrams describe the same information, and can be transformed into one another without difficulty.

However, collaboration diagrams use the free-form arrangement of objects as used in Object diagrams. In order to maintain the ordering of messages in such a free-form diagram, messages are labeled with a chronological number. Reading a Collaboration diagram involves starting at message 1.0, and following the messages from object to object.

Here is an example of the (simple) Restaurant System. Notice how you can follow the process from object to object, according to the outline below:

• 1. Order Food
• 1.1 Order Food
• 2. Serve Wine
• 3 Pickup
• 3.1 Serve Food
• 4 Pay

Statechart Diagram

See Harel Statechart.

Activity Diagram

Activity diagrams represent the business and operational workflows of a system. An Activity diagram is a variation of the state diagram where the "states" represent operations, and the transitions represent the activities that happen when the operation is complete.

Deployment Diagram

Deployment diagrams serve to model the hardware used in system implementations and the associations between those components. The elements used in deployment diagrams are components, nodes and associations.

Criticisms of UML

Although UML is a widely recognized and used standard, it has always been criticized for having imprecise semantics, which causes its interpretation to be subjective. Another problem is that UML doesn't apply well to distributed systems. In such systems, factors such as serialization, message passing and persistency are of great importance. UML lacks the ability to specify such things. For example, it is not possible to specify using UML that an object "lives" in a server process and that it is shared among various instances of a running process.

Extensions to UML

• Business Extensions are described by Magnus Penker and Hans-Erik Eriksson in Business Modeling with UML (ISBN 0471295515). Ovidiu S. Noran at the Griffith University compares UML and IDEF in Business Modelling: UML vs. IDEF.

See also

• List of UML programs
• Integrated Development Environment
• Object Constraint Language (OCL)
• Object-modeling technique
• Object-oriented
• XMI, a standard XML-based format for exchanging UML models.

External links

• UML Resource Page of the Object Management Group. Contains among other information the UML specification.
• Unified Modeling Language at FOLDOC
• A free (only SAMPLE COURSEWARE) UML tutorial
• Another free UML tutorial
• Article Database modeling in UML from Methods & Tools
• Article Death by UML Fever on the occasional mis-use of UML, from ACM Queue